Spatial Light Modulators (SLMs) are widely used in the industry for video monitors, graphic displays, projectors, and hard copy printers. SLMs are devices that modulate incident light in a spatial pattern corresponding to an electrical or optical input. The incident light may be modulated in its phase, intensity, polarization, or direction. This light image is directed and focused to a screen in the case of a projector, video monitor or display, or is ultimately focused on a light sensitive material, such as a photoreceptor drum, in the case of a xerographic printer.
The light modulation may be achieved by a variety of materials exhibiting various electro-optic or magneto-optic effects, and by materials that modulate light by surface deformation. Other spatial light modulators may include tiny micro-mechanical devices comprising an array of positionable picture elements (pixels). The light image can be colored if it is to be displayed on a screen of a projector, monitor, or a television and the like. This coloring is typically done in one of two ways, either using non-sequential color systems, or using sequential color systems. A non-sequential color system simultaneously generates multiple colors of light, such as red, green and blue light. An example of a non-sequential color system is discussed in commonly assigned U.S. Pat. No. 5,452,024, issued Sep. 19, 1995, entitled "DMD Display System", the teachings included herein by reference. In sequential color systems, color images are generated by sequentially projecting colored light, i.e. red, green and blue light, in a single image frame, which typically lasts 1/60 of a second for a 60 Hertz system. Sequential color systems typically utilize a color wheel that is partitioned into a plurality of colored segments, such as a red, green, and blue segment, or multiples/combinations thereof. An example of a sequential color system is disclosed in commonly assigned U.S. Pat. No. 5,448,314, issued Sep. 5, 1995, entitled "Method and Apparatus for Sequential color Imaging", the teachings included herein by reference.
A recent innovation of Texas Instruments Inc. of Dallas, Tex. is an imaging system using an SLM having an array of individual micro-mechanical elements, known as a digital micromirror device (DMD), also referred to as a deformable mirror device. The DMD is a spatial light modulator suitable for use in displays, projectors and hard copy printers. The DMD is a monolithic single-chip integrated circuit, comprised of a high density array of 16 micron square deflectable micromirrors on 17 micron centers. These mirrors are fabricated over address circuitry including an array of SRAM cells and address electrodes. Each mirror forms one pixel of the DMD array and is bi-stable, that is to say, stable in one of two positions. A source of light directed upon the mirror array will be reflected in one of two directions by each mirror. In one stable "on" mirror position, incident light to that mirror will be reflected to a projector lens and focused on a display screen or a photosensitive element of a printer, and forms an image of the mirror/pixel. In the other "off" mirror position, light directed on the mirror will be deflected to a light absorber. Each mirror of the array is individually controlled to either direct incident light into the projector lens, or to the light absorber. In the case of a display, a projector lens and a light prism ultimately focus and magnify the modulated light image from the pixel mirrors onto a display screen and produce a viewable image. If each pixel mirror of the DMD array is in the "on" position, the displayed image will be an array of bright pixels.
For a more detailed discussion of the DMD device, cross reference is made to U.S. Pat. No. 5,061,049 to Hornbeck, entitled "Spatial Light Modulator and Method"; U.S. Pat. No. 5,079,544 to DeMond, et al, entitled "Standard Independent Digitized Video System"; and U.S. Pat. No. 5,105,369 to Nelson, entitled "Printing System exposure Module Alignment Method and Apparatus of Manufacture", each patent being assigned to the same assignee of the present invention and the teachings of each are incorporated herein by reference. Gray scale of the pixels forming the image can be achieved by pulse width modulation techniques of the mirrors, such as that described in U.S. Pat. No. 5,278,652, entitled "DMD Architecture and Timing for Use in a Pulse-Width Modulated Display System", assigned to the same assignee of the present invention, and the teachings of which are incorporated herein by reference.
In non-sequential color systems, three (3) DMD arrays can be used to modulate colored light, one each for red, green, and blue light, as disclosed in the commonly assigned U.S. Pat. No. 5,452,024, issued Sep. 19, 1995, titled "DMD Display System", the teachings of which are included herein by reference. In contrast, a sequential color system requires only one such DMD device, with the red, green, and blue light being sequentially modulated and reflected by the single DMD array to a screen. The need for three such arrays in the non-sequential color system triples the requirement for the DMD arrays, and attendant hardware over the sequential color system, but offers increased display brightness. Thus, there is a trade off between the complexity, cost and performance of a non-sequential color system when viewed against a sequential color system.
In the case of a sequential color system, a single light source is typically used, such as disclosed in U.S. Pat. No. 5,101,236 to Nelson, et al, entitled "Light Energy Control System and Method of Operation", assigned to the same assignee as the present invention and the teachings of which are included herein by reference. These lamps may typically be comprised of a xenon or metal halide arc lamp, or lasers. This arc lamp may be powered by an AC or DC power source.
In conventional display systems, the video frame rate or refresh rate of the display is typically 60 hertz (Hz), or 60 frames a second. However, the incoming (source) data frame rate may vary from 60 frames a second. For instance, while a typical NTSC signal provides video frame data at 60 hertz, the video source frame rate for a PAL system is 50 hertz. Video or computer graphics data, meanwhile, is received at many other frame rates other than 60 hertz, typically being 72 hertz.
Typically, when the input frame rate deviates from 60 hertz in a 60 hertz sequential color display system, data must be discarded or filtered (e.g. averaged) before video is displayed due to the mismatch of bandwidths between the input source video and the output color sequential display rate. Rather than display video at 72 hertz for a 72 hertz input, the incoming data is discarded or filtered to maintain a 60 hertz video display frame rate. Discarding data will generate motion artifacts. Filtering data adds components and system cost. If incoming video data is received at a slow rate, operating in synchronism at a slower display rate, like 50 hertz, would generate a noticeable flicker in the display to the observer provided the screen is bright enough. Therefore, the data is averaged to maintain a 60 hertz display rate. Thus, retaining synchronous operation may not be sufficient for high quality video.
Additional problems arise in a DMD display system since data is displayed in a bit-plane format. One solution to the input/output frame rate mismatch, when doing color sequential displaying of data, is to asynchronously read the data from frame buffers. However, since the data displayed during any color segment may be from two different frames, a mixing of bit-plane information from these two frames occurs. This mixing of bit-plane can result in significant degradation of signal-to-noise ratio especially near MSB transition codes when using pulse width modulation techniques to determine the "on" time of the mirrors during one video frame. To see this, referring to FIG. 1, consider a DMD with 8-bits of color code, without bit-splitting, where the MSB is displayed during the entire first half of each color segment. If a pixel code 80 hex (midscale) from the source has a slight amount of noise, then 7 Fhex may be displayed during the first half of a color segment and 80 hex during the second half if two different frames are displayed during this color segment. A code of 00 hex would be displayed resulting in a half-scale error on the display as shown in FIG. 1. This is an extreme example, but even with bit-splitting during the color segments, the noise generation is significant when using an asynchronous frame buffer approach for frame rate conversion.
It is desirable to maintain the output color sequencing display frame rate of 60 hertz, even when the input source video frame rate is not at 60 hertz, without discarding, filtering or averaging video data. In addition, for DMD applications, it is desirable to eliminate the signal-to-noise degradation due to frame-to-frame bit-plane mixing during color segments without resorting to forcing the color sequencing rate to match the source video frame rate.